The Duke of Edinburgh was visited at The London Clinic today by his son, Prince Edward, and was expected to be visited tonight by his wife, Queen Elizabeth.
Prince Edward told reporters that his father was doing "well, thank you," as he left the hospital where his father has been since Thursday for what the palace described as an exploratory operation "following abdominal investigations."
Today, the palace released a statement saying the prince is "progressing according to plans."
"He is comfortable and in good spirits," read the statement from Buckingham Palace.
News of Philip's medical woes first broke last Monday when the duke pulled out of a scheduled reception to mark the 60 th anniversary of the queen's coronation because he felt unwell. He reportedly felt fine Tuesday and attended the queen's coronation anniversary service at Westminster Abbey along with the rest of the royal family.
Philip was taken to the hospital by car on Thursday after attending a garden party. He walked inside the facility on his own and was joking with staff there, sources told ABC News.
Buckingham Palace said today the prince will remain in the hospital for as long as two weeks. He is then expected to take an approximately two-month-long "period of convalescence."
Last year, Philip suffered recurring bladder infections and had to be hospitalized, including during the queen's Diamond Jubilee celebrations in June. Officials would not say whether today's hospitalization was related to Philip's earlier bladder or heart problems.
He was hospitalized for a chest infection in 2008, but recovered quickly. Officials revealed in 2007 that he had been suffering from a heart condition since 1992.
While Prince Philip received visitors at the hospital, his British subjects celebrated his birthday in the traditional way. Guns were fired in his honor at Edinburgh Castle, London's Green Park, the Tower of London, Cardiff Castle and Hillsborough Castle in Northern Ireland, the BBC reported.